Noninvasive positive pressure therapy is used for a variety of conditions including obstructive sleep apnea, central sleep apnea, and respiratory insufficiency. In respiratory insufficiency, the most common form of relevant therapy is bilevel therapy, in which a higher pressure (typically 15-25 cmH2O) is supplied during inspiration, and a lower pressure (typically around 5 cmH2O) is supplied during expiration. For central sleep apnea, an advanced therapy is adaptive servo-ventilation, where a complex pressure waveform is delivered, whose amplitude is constantly adjusted, spending much time around 5-10 cmH2O, but occasionally increasing to 20 cmH2O. For obstructive sleep apnea, advanced devices also vary the mask pressure during the night.
In all cases, the mask must be tightened sufficiently to seal against the highest pressure used. Consider, for example, a typical facemask, in which the area under the seal is 80 cm2.
If, for example, the highest pressure used is 25 cmH2O, then for a typical facemask, with a contact area of about 80 cm2, the total tension in the straps must exceed 25×80=2000 grams force (gF). (For a practical mask, which may not precisely fit the face, the force will need to be yet higher, in order to deform the mask and skin to seal. This additional conforming force will be discussed later.) Continuing the example, if the lowest pressure encountered is 5 cmH2O, then a total strap tension of only 5×80=400 gF is required to seal at this lowest pressure. Thus, in this example, if the mask seals at the highest pressure, then an excess force of 2000−400=1600 gF is applied at the lowest pressure. With a typical respiratory cycle, where 60% of the breath is at the lowest pressure, this excess force of 1600 gF is applied to the skin over the bony structures of the face for 60% of the respiratory cycle. In the case of adaptive servo-ventilation, where the highest pressure is required for only a very small part of the night, the excess force is applied for the greater part of the night.
This excess force causes considerable discomfort, and in not uncommon cases, actually causes breakdown of the skin, for example over the bridge of the nose.
Various methods for reducing this excess force have been proposed. All do mechanical work on the elastic elements of the mask and headgear, thus pulling the mask tighter as the mask therapeutic pressure increases, and releasing the mask as the mask pressure decreases. Such methods include a bladder in the top strap or a bladder (pneumatic pillow) between the rear strap and the back of the head (see, e.g., PCT Application No. PCT/AU03/01471 to inventors Michael Berthon-Jones et al. filed Nov. 6, 2003, incorporated herein by reference in its entirety), or a bellows in the body of the mask itself (see, e.g., U.S. Pat. No. 6,772,760 and U.S. patent application Ser. No. 10/655,622, filed Sep. 5, 2003, each incorporated herein by reference in its entirety). These approaches supply an external source of energy, derived from the varying therapeutic gas pressure itself, to provide the energy required to counteract the distortion of the headgear, mask structures, and facial and nuchal tissues as the pressure rises.
Another way of reducing the discomfort and improving the seal of a respiratory mask is to use a quasi-toroidal air filled closed bladder as the sealing element. A traditional anesthetic face mask has a relatively thick walled and non-compliant quasi-toroidal air filled bladder. It requires great force to deform such a thick walled bladder to fit the patient's face. This is acceptable in an anaesthetized patient but not in a sleeping patient. A greatly improved closed quasi-toroidal bladder is very thin walled and compliant in the skin contact region, increasing gradually in thickness elsewhere, and supported by a rigid frame, as taught in PCT application No. PCT/AU2004/00563, filed Apr. 30, 2004, incorporated herein by reference in its entirety. A particular advantage of the thin-walled compliant skin contact region is that it provides medially directed pressure onto the nasal bones, whereas other prior art generally supplies only a posteriorly directed pressure, resulting in either leaks into the eyes or excessive force on the bridge of the nose or both. Such an improved bladder can be advantageously combined with any of the previously described methods which derive energy from the varying mask therapeutic pressure. A first disadvantage of such a thin-walled bladder is it has a tendency to slowly deflate during the night, due to leakage and/or diffusion of air through the very thin wall. A second disadvantage is that it does not, of itself, have a mechanism for deriving energy from the varying mask therapeutic pressure, and therefore cannot supply the energy required to counteract the distortion of headgear, mask structures, and facial and nuchal tissues as therapeutic pressure rises.
Accordingly, a need had developed to address the potential disadvantages of the prior art masks described above.